1. Field of the Invention
The present invention relates to a process for preparing compound semiconductor films and especially III-V or II-VI compound semiconductor films.
2. Description of the Prior Art
In order to grow compound semiconductor films over the surfaces of substrates, methods have been devised and demonstrated including the metallorganic Chemical Vapor Deposition (MOCVD) method, molecular beam epitaxy (MBE), atomic layer epitaxy (ALE), molecular layer epitaxy (MLE) and so on.
The MOCVD method is disclosed in detail by, for instance, H. M. Manasevit et al. (J. Electrochem. Soc:. 120 (1973), 569). According to this method, as shown in FIG. 1, organometallic compounds and hydride compounds are transported by a hydrogen carrier gas over the heated surface of a substrate so that a compound semiconductor layer is formed over the surface of the substrate by thermal decompositions. For instance, in the case of GaAs which is one of the III-V compound semiconductors, trimethyl gallium (CH.sub.3).sub.3 Ga which is an organometallic compound of Ga and arsine which is a hydride compound of As are introduced over the surface of a heated substrate 1 disposed in a reaction vessel 3, decomposed and diffused into a velocity boundary layer 2 defined adjacent to the substrate 1 so that completely decomposed Ga and As atoms adhere to the surface of the substrate 1, thereby growing a GaAs crystal. While trimethyl gallium (TMG) is almost not decomposed outside of the boundary layer 2, it is almost 100% decomposed over the surface of the substrate 1 so that a trimethyl gallium concentration gradient exists in the boundary layer 2. The growth rate is approximately in proportion to the concentration gradient, so that the growth rate becomes higher at the upstream side at which the boundary layer 2 is thin and the growth rate is gradually decreased downstream as shown in FIG. 2. Thus, the most serious defect of the MOCVD method is that the thickness of the grown layer over the surface of a substrate is not uniform. Meanwhile, research and development of semiconductor devices having an extremely fine structure such as a quantum well structure, a superlattice structure and so on is increasingly being carried out and now demands an epitaxial technique cabable of controlling a single atomic layer. According to the MOCVD mehtod, the growth rate is determined by a raw material supply rate and, as described above, the thickness of the grown layer is not uniform over the whole surface of a substrate so that it is next to impossible to control the single atomic layer over the wide surface of the substrate.
MBE is disclosed in detail by, for instance, L. L. Chang et al. (J. Vac. Sci. Technol. Vo. 10, No. 1, p. 11+ (1973)). Starting material elements, Ga and As in the case of GaAs, are heated in a high vacuum and GaAs crystal depositioned on the surface of a substrate. However, it is also difficult to control the single atomic layer over the wide surface of the substrate because of the non-uniform distribution in space of the molecular beam.
ALE (U.S. Pat. No. 4,058,430: 1977) was proposed by T. Suntola et al. as an improvement to MBE. According to this method, semiconductor elements are alternately supplied in the form of pulses, whereby single atomic layers are alternately formed over the surface of a substrate. This method is advantageous in that the film thickness can be controlled with an accuracy of an atomic layer, but in this method an element having a high vapor pressure tends to be vaporized so that lattice defects result, thus resulting in crystal degradation. T. Suntola et al. presented various reports on the formation of compound thin films by ALE in Thin Solid Films 65 (1980), pp. 301-307; and other publications, but formed compounds are almost all II-VI compound semiconductors and oxides and a III-V compound semiconductor; that is, GaP is disclosed only in one of the EXAMPLES of U.S. Pat. No. 4,058,430. The have not disclosed the formation of GaAs and AlAs. The reason why the application of this method to the formation of III-V compounds is less resides in the fact that the vapor pressure of a Group III atom is extremely low. That is, when Group III atoms are supplied, only adsorption occurs and the adsorption continues after one atomic adsorbed layer is formed.
MLE which can utilize ALE in the formation of III-V compound semiconductor films was proposed by Nishizawa et al. (J. Electrochem. Soc.; SOLID-STATE SCIENCE AND TECHNOLOGY: Vol. 132, No. 5, 1197-1200, 1985). One of the important features of MLE resides in that fact that since in response to the supply of Group III atoms, only adsorption results and the adsorption of only one atomic layer cannot be interrupted, the Group III atoms are supplied in the form of a molecule having a high vapor pressure. According to this method, the adsorption of the material molecules as a single molecular layer, the chemical reactions, removal of reaction products and the film growth proceed in the order named. FIG. 3 is a time chart for the introduction of material elements and FIG. 4 is a schematic view used to explain the growth mechanism. Arsine is introduced over the surface of a heated substrate 1 in a high vacuum and then is exhausted, whereby a single molecular layer of arsine is formed as shown at (a) in FIG. 4. Adsorbed arsine is subjected to thermal decomposition, whereby a single atomic layer of As is formed a shown at (b) in FIG. 4. Thereafter, trimethyl gallium is introduced and decomposed over the surface of the substrate, whereby a single molecular layer of GaAs is formed as shown at (c) and (d) in FIG. 4. The system is exhaused again, thereby discharging excess trimethyl gallium. When the above-described steps are repeated, single atomic layers are grown layer by layer.
However, the above-described method has the following defects:
(1) A single molecular layer of trimethyl gallium is adsorbed over the surface of a substrate and then is subjected to the thermal decomposition, whereby a single Ga atomic layer is formed. However, it is impossible to form a single Ga atomic surface which covers 100% of the surface of the substrate because of the three-dimensional interference and repulstion between trimethyl gallium molecules. As a result, it is impossible in principle to attain the thickness of a single atomic layer by one cycle.
(2) During the growth period, when TMG is introduced or exhausted or when AsH.sub.3 is exhausted, there occurs a case in which no As exists in the growth atmosphere. As a result, since As has a high vapor pressure, As leaves the grown layer, leaving vacancies which are filled by impurities. Therefore, a deep impurity level associated with the vacancies of As results. In addition, this method is carried out in a high vacuum so that hydrogen exists so that methyl radicals resulting from the decomposition of trimethyl gallium are not reduced by hydrogen and carbons in the methyl radicals react with the vacancies of As and are incorporated into the growth layer, thus becoming carbon acceptors. In the above-described reports, Nishizawa et al. indicate that the grown layer is of p type with carrier concentration of the order of 10.sup.19 cm.sup.-3 which proves the contamination of the growth layer with carbon.
(3) Since the decompositon of trimethyl gallium takes a relatively long time and in addition an exhaustion period is needed, one cycle time becomes longer and is on the order of 33 seconds. As a result, in order to obtain the growth layer of one micrometer, more than 30 hours are required, which is not satisfactory in practice.
Meanwhile the process for fabrication of GaAs integrated circuits (ICs) such as GaAs MESFETs demands a technical process capable of forming low-resistance ohmic electrodes. Therefore, a process for introducing a high concentration n.sup.+ layer (&gt;5.times.10.sup.19 cm.sup.-3) is required. To this end, an ion implantation method and an epitaxial growth method have been devised and demonstrated. The ion implantation method is suitable for fabricating fine structures, but has the problems that an introduction of defects is caused by the ion implantation and that an annealing step is further needed.
On the other hand, the metallorganic Chemical Vapor Deposition (MOCVD) method which is included in an epitaxial growth method is excellent because in the case of formation of a III-V compound semiconductor film, the film thickness can be controlled with a relatively high degree of accuracy of on the order of 100 .ANG. and because it is adapted for mass production. This method will be described in connection with the doping of Si into GaAs which is a typical III-V compound semiconductor. An organic gallium compound sucha s trimethyl gallium, arsine (AsH.sub.3) which is a hydride of Group V element, silane (SiH.sub.4) which is a hydride of a Group IV element and a hydrogen carrier gas are introduced over the surface of a substrate which is heated at high temperatures (650.degree.-750.degree. C.), whereby an n-type GaAs layer doped with Si resulting from thermal decomposition is formed. However, so far there has been no report that the MOCVD method has succeeded in doping more than 5.times.10.sup.18 cm.sup.-3 (Duchemin et al.: J. Electrochem. Soc. 126, 1134 (1979)). The reason resides in the fact that the thermal decompositon of silane is difficult and that, in case of the supply of silane at a high concentration, reactions between the material gases occur so that the growth rate is decreased and the crystal surface is degraded. In like manner, for germane (GeH.sub.4) (Duchemin et al.), hydrogen selenide (H.sub.2 Se) or hydrogen sulfide (H.sub.2 S) which are hydrides of Group VI elements, n-type doping beyond 5.times.10.sup.18 cm.sup.-3 has been impossible. (Bass and Oliver: International Simposium on GaAs and Related Compounds, St. Louis, 1976, edited by L. F. Eastman (Inst. Phys. Conf. Ser. No. 33b, London 1977), p. 1). The reason is that when supplying a doping gas at a high concentration, it reacts with material molecules in the vapor phase, resulting in a decrease in growth rate and the degradation of surfaces Even for p-type doping with dimethyl zinc and diethyl zinc which are organometallic compounds of Group II elements, it was impossible to dope beyond 10.sup.19 cm.sup.-3 (Aebi et al.: J. Cryst. Growth 55 (1981) 517). The reason is that when introducing of a doping gas at a high concentration, it reacts with material molecules in the gas phase, resulting in a decrease in growth rate and surface degradation.
As a method for doping impurities only into a single atomic layer and forming a doped III-V compound semiconductor thin film, an atomic layer doping method utilizing the MBE and MOCVD methods may be used. With an example in which GaAs which is a typical III-V compound semiconductor is doped with a single atomic layer of Si, the MBE and MOCVD methods are explained.
According to the MBE method, Ga and As are deposited on the surface of a GaAs substrate which is heated in a high vacuum. Thereafter, the Ga flux is interrupted so that the growth of GaAs crystals is stopped and Si flux is supplied with As flux. Next, Ga and As are deposited again, whereby a GaAs crystal doped with Si in a single atomic layer can be obtained. The maximum surface density available so far by this method is 5.times.10.sup.12 cm.sup.-2 Sasa, S. Muto, K. Kondo, H. Ishikawa and S. Hiyamizu: Jpn. J. Appl. Phys. 24 (1985)L602). However, the supply of As and Si flux must be continued for longer than 300 seconds in order to obtain the high surface density as described above, so that the Si atom doping efficiency is extremely low.
According to the MOCVD method, an organic gallium compound and arsine together with the hydrogen carrier gas are supplied to the surface of a heated GaAs substrate. Next, the supply of the organic gallium compound is interrupted and then arsine and silane are supplied. Thereafter, arsine and the organic gallium compound are supplied again, whereby a GaAs crystal doped with Si in a single atomic layer can be obtained. However, according to this method, the maximum surface density is extremely low and is less than 10.sup.12 cm.sup.-2 (H. Ohno, E. Ikeda and H. Hasegawa: Jpn. J. Appl. Phys 23 (1984) L369). The reason is that since the growth temperature is as high as 700.degree. C. so that Si atoms are diffused during the crystal growth, the diffusion of Si atoms cannot be interrupted only ofter diffusion into a single atomic layer.
In the case of doping Si into GaAs by both the MBE and MOCVD methods, doping is carried out with excess As atoms; that is, doping is effected always into an As stabilized surface. When Si is doped into a Ga surface by a conventional method, the As beam or the supply of AsH.sub.3 is interrupted so that As atoms having a high vapor pressure evaporate from the undersurface of the Ga surface, adversely affecting the surface smoothness of a thin film. As a result, it has been impossible to dope a Ga surface with Si.
Meanwhile, according to the MOCVD method, as reported by T. K. Kuech et al. (Appl. Phys. Lett 44 (1984)986), the SiH.sub.4 doping efficiency is significantly dependent upon the growth temperature and the higher the temperature, the higher the carrier concentration becomes. The experimental results show that when a temperature distribution exists in the surface of a substrate, uniform doping in the surface cannot be attained as reported by Kuech et al. In addition, in the case of doping at higher concentration, the growth temperature must be raised. As a result, a profile cannot be distinctly defined due to the diffusion of doping atoms.
As described above, according to the MBE and MOCVD methods, the adsorption power of doping molecules is weak so that elimination results. Consequently, doping concentration is limited.
According to the ALE method proposed by Suntola and the MLE method proposed by Nishizawa et al., the purity of an undoped layer is considerably poor and the investigation of doping was not made so that the present invention is not comparable to the ALE and MLE methods.